1. Field of the Invention
The invention generally relates to a device which minimizes reflectivity caused by reflected and diffracted waves within a test chamber. Specifically, the invention is a wave absorber assembly comprising a plurality of first wedge-shaped absorbers and a plurality of second wedge-shaped absorbers which are disposed so as to form a continuous and smoothly changing v-shaped pattern along the interior surface of planar or non-planar walls of an anechoic chamber. The arrangement of v-shaped absorbers improves control over reflectivity within a chamber so as to better control primary and secondary scattering effects which in turn improves quiet zone reflectivity and reduces quiet zone clutter.
2. Background
A typical anechoic chamber for electromagnetic compatibility (EMC), far-field (FF) antenna, or radar cross section (RCS) measurements includes a metallic enclosure with internal surfaces covered by an absorbing material. An anechoic chamber also contains a direct illumination source antenna, a compact range reflector system, and/or a feed, the latter employed within a tapered chamber. A chamber could also include positioning equipment to rotate an antenna under test so as to acquire pattern data, to rotate a device under test for electromagnetic compatibility measurements, or to acquire incident collimated wave signals during RCS measurements. The primary purpose of an anechoic chamber is to create a test zone surrounding the antenna or device under test wherein the electric field is as uniform as possible and reflections are minimized.
The performance of a test zone within an anechoic chamber depends on the geometry, size of the test zone as determined by the dimensions of the antenna or device under test, source antenna performance, separation between source and antenna/device under test, and absorption properties and grades of the absorbing materials lining the interior walls of the chamber.
For a conventional antenna/test item system, the dimensions of an anechoic chamber and choice of source antenna depend on the size of the test zone. Field uniformity within the test zone is primarily determined by the electric field amplitude and phase taper of the wave radiated into the test zone by the source antenna. Field uniformity is improved by increasing the separation distance between the source and antenna/device under test so as to decrease the amplitude and phase taper. Typically, the separation distance is greater than 2*D2/λ, where D is the maximum aperture dimension of the antenna or device under test and λ is the operating wavelength. For large aperture antennas operating at higher frequencies, this distance is very large. However, the level of reflections from walls, floor, and ceiling increases with chamber length, thus offsetting the benefit of separation distance in many applications. As a result, test zone performance is a trade-off between the dimensions of the test zone and chamber geometry.
For a compact range system illumination of a test zone, a nearly uniform electric field can be achieved within the test zone. However, reflections from the chamber enclosure, even when covered by an absorbing material, negatively contribute to the uniformity of illumination within the test zone.
When RCS measurements are performed, echo signals are produced by scattering from the device under test. These signals travel along direct and indirect paths to the source antenna. The echo signals via indirect paths are often reflected from the walls of the chamber, and as such could contaminate measurements when not removed. Neither absorbing material nor time range gating is capable of completely removing or eliminating all unwanted echo signals. As such, some unwanted signals ultimately arrive back at the source antenna at approximately the same time as the return from the device under test, thus establishing the chamber background clutter level which limits the lowest RCS signal measurable.
For a tapered chamber system, reflections from the tapered walls covered by absorbing materials negatively contribute to the test zone, as described herein.
The requirements for test zone illumination and for the overall performance of anechoic chambers are becoming ever more stringent in response to advancements in antenna, RCS, and EMC technologies. Numerous approaches are described in the related arts to enhance the performance of anechoic chambers.
The shaping of surfaces in an anechoic chamber for EMC and antenna measurements are described by Smith in U.S. Pat. No. 3,100,870, Buckley in U.S. Pat. Nos. 3,113,271 and 3,120,641, Hemming in U.S. Pat. No. 4,507,660, Sanchez in U.S. Pat. No. 5,631,661, Shibuya in U.S. Pat. No. 4,906,998, Kogo in U.S. Pat. No. 4,931,798, and Berg et al. in U.S. Pat. No. 6,008,753. Shaping facilitates a reflecting screen or wall that deflects reflected energy from the chamber wall, typically comprising an absorber element and metallic backing, away from the test zone, thus reducing reflectivity levels within the test zone, minimizing or avoiding specular zones on the side walls, and avoiding the need for absorbing materials along some areas of a chamber. For example, Shibuya describes the use of shaping to deflect unwanted radiation towards the open walls outside of a chamber and away from the test zone. In another example, Kogo describes the use of shaping so that reflections from walls are focused at some place within the chamber outside of the test zone, usually behind the test zone. Insertion of an absorption “ball” at this location further reduces echoes within the chamber. In another example, Berg et al. describes the use of shaping so that the echo signals reflected by the device under test, and subsequently by the side walls, are focused at a point located behind the source antenna, thus minimizing the clutter level in RCS measurements.
Other inventions in the related arts have focused on the design of absorber elements. Most absorber designs assume a flat metallic backing along the absorber and ignore detailed consideration of the scattering effect. For example, Hemming et al. in U.S. Pat. No. 4,496,950 describes shaped absorber components to create normal or close to normal incident angles, thus improving absorption. In another example, Burnside et al. in U.S. Pat. No. 6,437,748 suggests the use of an R-card or “Chebyshev” pattern. This latter approach assumes the absorber is highly efficient and performance does not depend on a metallic backing plate.
The inventions described above are broadly based on the principles of Geometrical Optics approximation. As applied to anechoic chamber design, these principles assume that the walls of a chamber, even when covered by a highly efficient absorbing material, reflect a maximum signal level at about the same angle as the arrival angle of the incident wave. As such, the related arts have focused on minimizing the specular reflection of waves within an anechoic chamber into a test zone, while ignoring the causes and influences of diffracted waves. Furthermore, the related arts make no effort to control the diffracted waves in a primary scattered field resulting from the first impingement of a wave onto an absorber and the diffracted waves in a secondary scattered field resulting from the further impingement of the primary scattered field onto an absorber, test item, and/or source within a chamber. Accordingly, all presently known applications of absorbers within a test chamber do not adequately control the reflection and scattering of electromagnetic waves.
Therefore, what is required is an absorber assembly which accounts not only for specular reflections, but also for the primary and secondary scattered fields caused by diffraction within an anechoic chamber so as to better control the reflected fields therein.